Uranium and lead isotope ratios were measured on nine ore samples and four mineral separates. Major uranium mineralization occurred at Key Lake 1,350 + or - 4 m.y. ago. Uranium and lead were fractionated at 300 + or - 6 m.y. These ages are defined by a straight line on the concordia diagram determined by four of nine whole-rock ore analyses. The time of primary mineralization is the oldest reported for Proterozoic unconformity-related uranium deposits in the Athabasca Basin. Data obtained in this study show that lead loss (or uranium gain) has occurred in high-grade rocks and that lead gain (or uranium loss) has occurred in low-grade rocks.--Modified journal abstract.